14. (1 point) A mobile phone company offers a phone plan that allows 600 min of airtime per month. The revenue generated from this plan depends on the price of the plan according to the data in the table. Monthly Plan Price (S) Revenue ($1000s) 10 1180 15 20 25 30 35 1560 1740 1720 1500 1080 The first differences for this data imply a rate of change of revenue with respect to monthly plan price that is constant, increasing or decreasing?